Automobile manufacturers are actively working to develop alternative powertrain systems in an effort to reduce the level of pollutants exhausted into the air by conventional powertrains equipped with internal combustion engines. Significant development has been directed to electric vehicles and fuel cell vehicles. Unfortunately, these alternative powertrain systems suffer from several disadvantages and, for all practical purposes, are still under development. However, several different hybrid electric vehicles (HEV) have recently been offered for sale. These hybrid vehicles are equipped with an internal combustion engine and an electric motor that can be operated independently or in combination to drive the vehicle.
There are two types of hybrid vehicles, namely, series hybrid and parallel hybrid. In a series hybrid vehicle, power is delivered to the wheels by the electric motor which draws electrical energy from the battery. The engine is used in series hybrid vehicles to drive a generator which supplies power directly to the electric motor or charges the battery when the state of charge falls below a predetermined value. In parallel hybrid vehicles, the electric motor and the engine can be operated independently or in combination pursuant to the running conditions of the vehicle. Typically, the control strategy for such parallel hybrid vehicles utilizes a low-load mode where only the electric motor is used to drive the vehicle, a high-load mode where only the engine is used to drive the vehicle, and an intermediate assist mode where the engine and electric motor are both used to drive the vehicle. Regardless of the type of hybrid drive system used, hybrid vehicles are highly modified versions of conventional vehicles that are expensive due to the componentry, required control systems, and specialized packaging requirements.
Hybrid powertrains have also been adapted for use in four-wheel drive vehicles and typically utilize the above-noted parallel hybrid powertrain to drive the primary wheels and a second electric motor to drive the secondary wheels. Such a four-wheel drive system is extremely expensive and difficult to package.
Many owners of four-wheel drive SUV-type vehicles and pickup-type trucks enjoy the ability to tow relatively large trailers with these vehicles. For example, large travel trailers and/or construction equipment trailers may be towed by vehicles ranging in size from pickup trucks to 3500 size trucks. Furthermore, tow vehicles as large as Class 8 tractors are often coupled to trailers to transport cargo over the road. Depending on the load being towed, performance of the tow vehicle and trailer combination may be very different from the performance of the tow vehicle alone.
Most trailers that are towed by SUVs are equipped with wheels that are not operable to provide drive torque to the ground. The trailer wheels are typically not braked or include relatively crude braking systems that actuate the brakes based only upon a transfer of load to the trailer tongue during a deceleration of the tow vehicle. Furthermore, even though the tow vehicle may be equipped with traction control and/or stability control systems, little or no communication occurs between the tow vehicle control systems and the trailer. Lastly, the trailers that are equipped with brakes typically decelerate by converting kinetic energy into heat. The heat energy is not stored but simply vented to the atmosphere. Thus, a need exists to develop hybrid powertrains for use in towed trailers to improve energy efficiency, vehicle performance and handling.